Profiling single cells using single cell RNA sequencing (scRNAseq) is revolutionizing our understanding of development and disease. In this proposal, we will apply scRNAseq to create an atlas of cells that respond to biomaterials that induce divergent responses and serve as a model for tissue microenvironments of repair versus fibrosis. The proposed research aims to leverage single cell analysis to define key subpopulations in the lymphoid, myeloid and stromal fibroblasts response to biomaterial models of tissue fibrosis and repair. Minimally processed biological scaffolds induce a Type 2 immune response characterized by interleukin (IL)-4 and tissue repair, similar to muscle repair processes. Our preliminary data describes a Type 17 immune and senescent cell response to synthetic implants that induce fibrotic capsule formation in an IL-17-dependent manner. We also demonstrate the ability of scRNASeq to uncover new macrophage cell populations in biomaterial microenvironments. We hypothesize that by sorting cell subpopulations in the FBR in vivo, combined with single cell analysis, we will identify new and rare populations that will help elucidate mechanisms and provide new therapeutic targets to enhance tissue repair or reduce fibrosis. The following specific aims are proposed to accomplish this goal: Specific Aim 1. Identify and characterize lymphoid, myeloid, and fibroblast subpopulations isolated from synthetic and biological scaffold implants using single cell RNA sequencing analysis. Specific Aim 2. Computationally phenotype cell clusters both within and across cell types to define distinct subsets and interaction models using pseudotime analysis, RNA velocity, differential expression and gene set enrichment, cluster analysis to predict unique surface markers/combinations, and cell interactions analysis. Specific Aim 3. Define unique surface and intracellular markers from single cell analysis to identify subpopulations using standard experimental methods. Newly-identified immune and fibroblast subpopulations will be evaluated over time in male and female mice and results will be validated with diverse materials. The cell atlas created in the proposed research will enable future mechanistic studies and investigation into the potential broad applicability to wound healing, cancer and other tissue pathologies.